JPH09501491A - Nuclear fuel with improved fission product retention characteristics - Google Patents

Abstract

(57) [Summary] A nuclear fuel based on UO ₂ , ThO ₂ and / or PUO ₂ with improved properties for maintaining fission products. The fuel contains a metal that traps oxygen, such as Cr or Mo, and has a superstoichiometric oxide (U, Th) O _{2 + X} and / or (U, Pu) O _{2 + X} (0 <X ≤ 0.01) and includes metals capable of forming oxides with enthalpies of free formation less than or equal to. This traps the oxygen atoms released during the splitting of U, Th and / or Pu; in this way the fission product maintenance level can be increased and a high burning level of nuclear fuel components can be achieved.

Description

Detailed Description of the Invention                   Nuclear fuel with improved fission product retention characteristics   The present invention is UO₂, ThO₂And / or PuO₂Based on, improved splitting It relates to a nuclear fuel having a product maintenance property.   In the case of nuclear fuels based on oxides, especially uranium oxide, the use of these fuels One of the problems caused by this is that during operation of the reactor, the split gas in the fuel components is released. It is caused by This is because the fission product is the internal pressure of the sheath or can. And especially as the interaction between the fission products and the sheath or can is limited. This is because it must be blocked in a component such as the fuel pellets of.   Therefore, at the moment, do not exceed the critical point where the release of fission gas is significant. In addition, the burning rate of nuclear components is limited to 50 GWj / t in U.   However, especially operators of electric nuclear reactors such as pressurized water reactors (PWRs) Is the uranium dioxide contained in the rod to achieve a minimum value of 60-70 GWj / tU. To optimize the control of nuclear fuel by increasing the burning rate of pellets. ing.   Research to improve the above has, to date, increased the size of uranium dioxide particles. Has been used to generate the gas emitted by the irradiated large particle fuel. Body mass proved to be less than emitted by irradiated small particle fuel This is because. The operation of forming a precipitate in nuclear fuel also fixes the fission gas to the precipitate. Is used to   In order to increase the size of uranium dioxide particles, iO, NbO, CrO, Al Additives such as O, VO and MgO can also be dissolved to activate their crystal growth. Adding to the uranium dioxide powder, which is sometimes sintered, The above properties are maintained in the uranium dioxide solution and are not reduced to metal atoms. It is possible if the sintering is carried out in a wet hydrogen environment. You. Use of additives such as those described above to obtain large microstructure particles. For example, see Killeen, Journal of Nuclear Materials, 88, 1980, pp. 177-184. , Sawbridge et al., Report CEGB RD / B / N 4866, July 1980 and Radford et al., Scient. ific Paper 81-7D2-PTFOR-P2, 1981. However, this Thailand The use of certain additives in the , Which is undesirable for the maintenance of fission products, and It is not possible to take full advantage of the black structure large particles.   Another way to improve nuclear fuel retention is to ensure fission product fixation. In order to separate the second stage nanoprecipitate into the uranium dioxide particles. Consists of scattering. This type of nanoprecipitation is described by Sawbridge et al., Journal of Nu. Clear Materials, 95, 1980, pp. 119-128 and FR-A-2026251 It may be composed of a magnesium oxide-containing material such as   The present invention has been described so far in order to improve the retention rate of fission products in nuclear fuel. Use a different method than the one listed. The method comprises uranium and / or plutonium Consisting of trapping oxygen atoms released by atom splitting, at 2 O / U of fuel where M = U + Pu or U + Th or U + Pu + Th ( Th, Pu) or O / M stoichiometry, thus increasing the diffusion coefficient in the fuel. It is intended to avoid the addition and the reduction of its thermal conductivity.   Therefore, increasing the diffusion coefficient is a mechanism leading to the accumulation of fission products at the grain boundaries. Yes, and eventually the release of the fission product occurs. Similarly, the reduction in fuel thermal conductivity is , Increasing fuel temperature for the same linear power, resulting in fission on the one hand It has the effect of reducing the solubility of the substances and, on the other hand, helping them diffuse. It is profitable.   The present invention also relates to UO₂, ThO₂And / or PuO₂Based ceramics Nuclear Method for improving fission product retention in fuel, ceramic nuclear fuel At the operating temperature T of the reactor, the chemical formula (U, Th) O_{2 + x}And / or (U , Pu) O_{2 + x}And in the above chemical formula, x is super stoichiometry such that 0 <x ≦ 0.01 Free formation enthalpy lower than free formation enthalpy of oxide at the same temperature T Oxygen can be trapped by forming an oxide with a pea, It comprises at least one metal.   The use of such additives results in the above mentioned O / U (Th or Pu) or the O / M ratio can be kept at a value of 2, which would allow diffusion It is possible to avoid increasing the coefficient and maintain a low value, and avoid decreasing the thermal conductivity of the fuel Become. Therefore, a high fission product retention rate is obtained.   This procedure is UO₂And / or PuO₂And / or ThO₂Increase particle size And known methods of forming precipitates for the fixation of fission gases. This is very interesting and can improve the performance characteristics of the fuel. It is possible.   In the case of fuels based on uranium dioxide, superchemistry with 0 <x ≦ 0.01 Stoichiometric oxide UO_{2 + x}Enthalpy of free formation of the Journal of Nuclear Materials , 130, 1985, pp 473-488, as represented by the oxidation potential, Lindemer and Besm. Calculated based on Ann's law. In this case, as shown on page 480 of the above document, Oxidation potential of the above superstoichiometric oxide ΔG (O₂) Is expressed in J / mole by the following formula It is possible,     -360000 + 214T + 4RTLn [2x (1-2x) / (1-4x)²] In the above equation, R is the gas molar constant, T is the temperature in Kelvin, and x is the above As defined.   Furthermore, for fuels based on uranium dioxide, included in the fuel Metals cannot form oxides with an oxidation potential defined by Must               △ G (O₂) = RTLn (pO₂) In the above equation, R is the gas molar constant and T is the reactor operating temperature in Kelvin. p (O₂) Is the partial oxygen pressure, which is UO according to Lindemer and Besmann's law._{2 + x}To It is equal to or smaller than the value evaluated above. An example of a suitable metal is Cr , Mo, Ti, Nb and U.   The present invention provides that at least one metal capable of trapping oxygen is dispersed, UO having the above characteristics₂, ThO₂And / or PuO₂Based ceramics It also relates to fuels for nuclear reactors that contain oxidative substances.   According to the invention, the oxide-based ceramic material is UO₂, ThO₂, PuO₂ Alternatively, a mixture thereof, a mixed oxide UO₂-PuO₂Or UO₂-ThO₂ , UO such as rare earth oxides₂Mixed oxides and other oxides based on Kuha PuO₂Can be made of a mixed oxide based on.   As mentioned above, the ceramic material is UO.₂Based on the above dispersed gold Genus is UO_{2 + x}Can form oxides with an oxidation potential lower than that of Preferably.   Generally 60 GWj / t^-1In order to obtain the burning rate of It accounts for 0.1 to 2% by weight of the quality. Preferably, the metal is chromium and the fuel It constitutes 0.1-1% by weight of the quality, more preferably 0.2-0.5% by weight.   Furthermore, in order to increase the size of the fuel particles and / or to link the fission products. The fuel material is TiO 2 in order to promote the lapping.₂, Nb₂O_Five, Cr₂O_Three, Al₂O3, V₂O_FiveAnd additives such as MgO can be included to improve other properties. For example, SiO₂Other additives such as can be included as well.   The fuel material of the present invention is in the form of a metal or oxide or oxygenated compound. Conventionally, by adding a ceramic substance powder to be melted to some form of metal, Can be prepared by the sintering or melting method.   In the first case, after compacting the powder by cold compression, so as not to oxidize the metal, For example, sintering is performed in a dry hydrogen environment having a water content of 0.05% by volume or less. .   In the second case, UO₂, ThO₂And / or PuO₂Get particle size increase at the same time If desired, UO at sintering temperature₂, ThO₂, PuO₂The oxide inside Is an oxygenated compound, and its amount exceeds the solubility limit. In order to keep the oxide during sintering and activate crystal growth, it may or may not exceed , Wet or moistened hydrogen (eg having a water content of more than 1% by volume) Sintering is performed after powder molding by low temperature compression under humidified hydrogen). Burning After binding, the sintered material reduces the oxides or oxygenated compounds to metals. In addition, for example, dry hydrogen having a water content of 0.05% by volume or less en) go through the reduction process below.   In the second case, one of the following microstructured large particles (diameter 40 μm) can be obtained: If the amount of added oxide or oxygenated compound is below the solubility limit, , Nanometer metal precipitation (diameter less than 100 nm), If the amount of oxide or oxygenated compound added exceeds the solubility limit, , Nanometer metal precipitation (diameter less than 100 nm) and micrometer metal precipitation (Diameter 0.3 μm or more).   However, in the second case, the powder compaction followed by cold compaction results in a large grain structure. If you do not want to obtain the structure at the same time, sinter the oxide or oxygenate at the same time. In order to reduce the compound to a metal, it has a water content of, for example, 0.05% by volume or less. It is performed in a dry hydrogen environment. In this case, micrometer metal precipitation within the particles Obtained (diameter 0.3 μm or more).   In all of these cases, cold compression, such as forming pellets, is used. For example, the powder can be molded by conventional multi-axial compression under a pressure of 200 to 700 MPa. Can be done by any method.   A temperature of 1600 to 1750 ° C. is usually used for the heat treatment.   If there is an additional reduction heat treatment, the heat treatment is performed at a temperature of 1300 to 1750 ° C. It can be carried out.   Ceramic material powders containing metal additives in the form of oxides or oxygenated compounds are , Mixing powders of constituents, or adding additives in the form of salts in solution. Atomization-drying method using a lip, or It can be prepared by coprecipitation of a uranium salt and a salt of an additive.   Thus, the method of the present invention not only provides the oxygen trapping ability of the added metal, UO₂Metals to activate crystal growth and improve the retention of fission products in the fuel It is very interesting because it makes it possible to take advantage of the properties of oxides .   Other characteristics and advantages of the present invention include the following from the non-limiting description below. It can be known by referring to the attached drawings. Fig. 1, 2 & 3 Changes in the oxidation potential (kJ / mol) of various oxygenated compounds with temperature (℃) ) As a function of FIG. 4 is a micrograph of a fuel material according to the present invention. Figure 5 Microscope showing oxygen atoms trapped in the fuel material according to the present invention It is a microscopic photograph. Figure 6 Objective of structural comparison of prior art fuel materials subject to controlled oxidation It is a micrograph given by. 7 is a micrograph of a fuel material according to the present invention having a small particle structure. You. Figure 8 Micrograph of fuel material according to the invention with large microstructured particles Is true.   Figure 1 shows UO₂The oxidation potential calculated based on Lindemer and Besmann's equation in kJ / mole Represents and superstoichiometric oxide UO_{2 + x}And superstoichiometric oxide UO_2-xAlso for And as a function of temperature expressed in ° C.   Figure 2 shows Cr / Cr₂O_ThreeChange of pair oxidation potential (kJ / mole) as a function of temperature (℃) , Over the entire temperature range of interest, the oxidation potential of the above oxide is The superstoichiometric oxide UO of FIG._{2 + x}It is shown to be smaller than that of.   Figure 3 shows MoO₂The change in the oxidation potential (kJ / mole) of p. Superstoichiometric oxide UO at temperature_{2 + x}That it is still smaller than It is shown.   As a result, these two components are₂Oxygen for fuel materials based on Suitable as a metal that can be trapped, the example below shows the two Minute UO₂It details the use with. In all examples, the average particle size UO of 0.5 to 100 μm₂Use powder.Example 1   In this example, UO combined with a Cr micrometer metal precipitate.₂Pele Prepare the kit.   UO₂100 g of powder and 0.1 g of Cr metal powder having an average particle size of 2 μm or less After mixing and, the mixture was made into pellet form by uniaxial compression at 350 MPa. However, the matrix (mold) was oiled in a hydraulic press. Pele above Put in a molybdenum boat and sinter under dry hydrogen at 1700 ° C. for 4 hours. . Thus UO with micrometer metal precipitation of Cr₂Gives microstructured small particles I got it.   FIG. 4 is a micrograph showing the above structure at a magnification of 600 times. Above microscope In the photograph, metal precipitation (white particles) between particles or within particles is not observed. Obviously, the electron diffraction pattern confirmed the metallic properties of these inclusions.   To verify the action of the above fuels to trap oxygen, in the case of pure oxygen 0.01 volume, which is a condition that can achieve an average O / U ratio of 2.024. The heat treatment at 700 ° C. in a helium atmosphere containing 10% oxygen results in a pellet. A controlled oxidation of the oxidase was performed.   FIG. 5 is a microscope image showing the structure of the fuel material that has undergone the above-mentioned oxidation at a magnification of 400 times. Is true. Fuel substance traps oxygen, UO₂Only the phase that has already obtained the matrix I know there isn't.   For comparison purposes, FIG. 6 is similar to Example 1 but without the addition of chromium. And under similar controlled oxidation to obtain an average O / U ratio of 2.024 The obtained pellet of uranium dioxide is shown. Figure 6 shows UO₂Not in the matrix Required U_FourO₉Is present.   Thus, comparing FIGS. 5 and 6, UO₂U_FourO₉ You can know the effect of preventing the conversion to.Example 2   In this example, UO with micrometer metallic Cr precipitates₂Micro structure small Preparation of uranium dioxide nuclear fuel pellets with particles was performed.   In this case UO₂100 g of Cr₂O_Three0.15 g of powder (particle size 2 μm or less) Mix and then formulate pellets from this mixture and dry as in Example 1 in a dry hydrogen atmosphere. Sintered under atmosphere.   In this case, the added chromium oxide is converted back to metallic chromium during sintering under dry hydrogen. UO to form microstructured large particles₂Activates the crystal growth of Absent. That is, microstructured small particles having a metal Cr precipitate were obtained. Figure 7 above The structure of the above is shown.Example 3   In this example, UO with metallic Cr precipitation₂Nuclei with microstructured small particles The fuel was prepared.   UO₂150 g, dissolved chromium salt: (NH_Four)₂CrO_Four0.6g and distilled water 250g Powders were prepared by atomizing and drying the slips containing. The powder obtained is Then chromium salt is added to Cr₂O_ThreeAlumina laboratory tube furnace (alumin a laboratory tubular furnace) under argon supply (300 ml / min) 400 ° C Calcination in an alumina boat for 2 hours.   In this case, the oxygenated compound of chromium is UO.₂Acting as a crystal growth activator Intactly reduced to chromium metal during sintering. Therefore, UO₂Mi A small amount of chromium structured particles was obtained together with a metallic chromium precipitate.Example 4   In this embodiment, the metal precipitation of Cr is nanometer and micrometer. UO₂Preparation of nuclear fuel with microstructured large particles was performed.   As in Example 3, (NH_Four)₂CrO_FourUO at 1.5g, ie 1700 ° C₂ Cr in₂O_ThreeCr exceeding the solubility limit₂O_ThreeAtomize dry using the content To prepare a powder. The powder obtained is treated according to example 3 to give alumina. In an experimental tubular furnace, under an argon supply (300 ml / min) at 400 ° C., alumina bow Calcined in the oven for 2 hours. Then, as in Example 1, this was multi-axial at 350 MPa. It was in the form of pellets by compression. After that, turn this into chromium oxide form Keep it, UO₂Humidified with 1.7% by volume of water to facilitate particle size increase Sintering was performed at 1700 ° C. for 4 hours in a hydrogen environment.   After sintering, Cr₂O_ThreeWater content of 0.05 to reduce oxides to metallic chromium Annealing treatment was carried out at 1300 ° C. for 5 hours under dry hydrogen with a product%.   Cr during sintering₂O_ThreeTo keep it in the oxide form for crystal growth. It becomes possible to use it as an activator, and by this method, microstructure large particles are obtained. Cr after the subsequent annealing treatment under dry hydrogen₂O_ThreeIs reduced to metallic chromium, followed by It is now possible to obtain nanometer and micrometer metal precipitates.   The microstructure of the material obtained under these conditions is shown in FIG.₂Large grain of The child 1 and the micrometer chromium inclusions 5 can be seen. Nanometer Chromium inclusions are shown by electron diffraction.Example 5   By atomizing and drying as in Example 3, except (NH_Four)₂CrO_Four0.2 g , Ie UO at 1700 ° C₂Cr in₂O_ThreeCr below solubility limit₂O_Three Powders were prepared with equivalent contents. Following this, the pellet form Compacted powder and obtained large microstructured particles to keep chromium in oxide form Therefore, sintering as in Example 4 was performed. Following this, Cr₂O_ThreeReturned to metallic chrome The annealing treatment as in Example 4 was performed for the purpose.   In this case, excess Cr, which forms micrometer metal precipitates during reduction,₂ O_ThreeDoes not exist, so UO₂Gold with microstructure large particles of Cr nanometer Obtained with genus precipitation.Example 6   This example adopts the same operation method as in Example 4, except that (NH_Four)₂CrO_Four1 . 5g and ultrafine SiO₂0.04 g to UO₂Soot containing 150g and 250g distilled water Used in the lip. The powder obtained by atomizing was the same as in Example 4. Under conditions, compressed into pellet form, then sintered in a humidified hydrogen environment, dry water It was annealed under the atmosphere. This allows UO with metallic chromium precipitation₂micro A silica phase was obtained at the structured large particles and grain boundaries.Example 7   In this embodiment, UO₂100g and MoO_Three0.6 g of the mixture of uranium metal Co-milling in a ball jar followed by this under the same conditions as in Example 1. The powder mixture was pressed and sintered into pellet form.   In this case, during sintering the molybdenum oxide is reduced to molybdenum,₂ It is not possible to activate grain crystal growth. Therefore, UO₂Micro structure Small particles were obtained with Mo micrometer metal precipitation.Example 8   UO₂150 g and ammonia paramolybdate (NH_Four)₆Mo₇O_{twenty four}・ 4H₂ A powder is obtained by atomizing and drying an aqueous suspension composed of O and 250 g of distilled water. Obtained. This powder was then treated as in Example 1. This makes UO₂Micro structure Small particles were obtained with micrometer, metallic Mo precipitation.

────────────────────────────────────────────────── ─── Continuation of front page    (71) Applicant: FRAMATOM             France 92400 Courbevoie Plus               Draukpol 1 Tourfi             A (72) Inventor Duo, Philip             France 38320 Avan Arede               Assel 10 (72) Inventor Pale, Veronique             France 26300 Burg-du-Pair             Jure Residence Le Cross-Doyse             Le Ryu de Dunkirk 1

Claims (1)

[Claims] 1. Enthalpy of free formation in ceramic fuel at operating temperature T of reactor At least capable of trapping oxygen by forming an oxide having It also contains one metal, and the above-mentioned enthalpy of free formation is superstoichiometric oxidation. Thing or chemical formula (U, Th) O_{2 + x}And / or (U, PU) O_{2 + x}In oxide , X is 0 <x ≦ 0.01, but the enthalpy of free formation at the same temperature T UO, which is equal to or smaller than₂, ThO₂And / or PuO₂ For improving the retention of fission products in Al-based ceramic nuclear fuel Law. 2. The above nuclear fuel is uranium dioxide UO₂Based on Characterized by being able to form an oxide having an oxygen voltage defined by the chemical formula Then               △ G (O₂) = RTLn (pO₂) In the above equation, R is the gas molar constant and T is the reactor operating temperature in Kelvin. pO₂Is a partial oxygen pressure and U is 0 <x ≦ 0.01 at the same temperature T. O_{2 + x}Is equal to or smaller than the oxidation voltage of               -360,000 + 214T + 4RTLn [2x (1-2x) / (1-4x)²] Is less than or equal to, where R, T and x have the meanings given above. The method according to claim 1, which is a characteristic. 3. The metal is selected from Cr, Mo, Ti, Nb and U. The method according to claim 1 or 2. 4. At least one capable of trapping oxygen to form an oxide The metal is dispersed and the enthalpy of free formation of the above oxide is a super stoichiometric oxide. (U, Th) O_{2 + x}And / or (U, Pu) O_{2 + x}Equivalent to the free enthalpy of formation of Or small, where x is at the temperature reached in the reactor UO such that 0 <x ≦ 0.01₂, ThO₂And / or puO₂Based on A fuel material for a nuclear reactor, characterized in that it comprises a ceramic. 5. The above metals have the chemical formula:               △ G (O₂) = RTLn (pO₂) Can form an oxide having an oxidation voltage defined by Is the molar constant of, and T is pO at the reactor operating temperature expressed in Kelvin.₂Is partial oxygen Pressure, below formula               360,000 + 214T + 4RTLn [2x (1-2x) / (1-4x)²] Less than or equal to, where R and T have the meanings given above, The substance according to claim 4, wherein x is 0 <x ≦ 0.01. 6. The metal is selected from Cr, Mo, Ti, Nb and U. The method according to claim 4 or 5, wherein 7. The metal is Cr, and the Cr content of the fuel material is 0.1 to 1% by weight. The substance according to claim 6, characterized in that 8. The chromium content is 0.2 to 0.5% by weight. The substance described in 7. 9. The ceramic material is UO₂And that it is a mixed oxide of rare earth oxides 9. A substance according to any one of claims 4 to 8 characterized. 10. Preparation of ceramic material powder containing metal capable of trapping oxygen A method, wherein the mixture is molded by cold compression and molded in this way 10. The powder according to claim 4, wherein the powder is sintered in a dry hydrogen environment. Item 1. A method for preparing a nuclear fuel material according to Item 1. 11. A metal oxide or oxygenated compound that can trap oxygen A method of preparing a powder of a ceramic material containing, comprising the steps of: The powder is shaped into a wet hydrogen ring to maintain oxides or oxygenated compounds. Sintered under ambient conditions and subsequently dried to reduce oxides or oxygenated compounds to metals. 10. The thermal reduction treatment is carried out in a hydrogen environment, according to any one of claims 4 to 9. A method for preparing a nuclear fuel material as described in the item. 12. A metal oxide or oxygenated compound that can trap oxygen What is claimed is: 1. A method for preparing a ceramic material powder containing And then sintered in a dry hydrogen environment at the same time from oxides or oxygenated compounds Claims characterized by performing a thermal reduction treatment in a dry hydrogen environment to reduce to metal Item 10. A method for preparing a nuclear fuel material according to any one of Items 4 to 9. 13. The ceramic powder is UO₂It is powder, It is characterized by the above-mentioned. 13. The method according to any one of 12 to 12. 14． The sintering is carried out at a temperature of 600 to 1750 ° C. Item 14. The method according to Item 13. 15. The heat treatment is performed at a temperature of 1300 to 1750 ° C. The method according to claim 11. 16. If the oxygenated compound of the above metal is ammonium chromate or paramolybde Ammonium acid salt according to any one of claims 11 to 15, characterized in that The described method.